Penetration of cefaclor into the interstitial space fluid of skeletal muscle and lung tissue in rats

Lung Collapse during Mini-Thoracotomy Reduces Penetration of Cefuroxime to the Tissue: Interstitial Microdialysis Study in Animal Models

Background: Single-lung ventilation facilitates surgical exposure during minimally invasive cardiac surgery. However, a deeper knowledge of antibiotic distribution within a collapsed lung is necessary for effective antibiotic prophylaxis of pneumonia. Patients and Methods: The pharmacokinetics/pharmacodynamics (PK/PD) of cefuroxime were compared between the plasma and interstitial fluid (ISF) of collapsed and ventilated lungs in 10 anesthetized pigs, which were ventilated through a double-lumen endotracheal cannula. Cefuroxime (20 mg/kg) was administered in single 30-minute intravenous infusion. Samples of blood and lung microdialysate were collected until six hours post-dose. Ultrafiltration, in vivo retrodialysis, and high-performance liquid chromatography-tandem mass spectrometry were used to determine plasma and ISF concentrations of free drug. The concentrations were examined with non-compartmental analysis and compartmental modeling. Results: The concentration of free cefuroxime in ISF was lower in the non-ventilated lung than the ventilated one, evidenced by a lung penetration factor of 47% versus 63% (p < 0.05), the ratio between maximum concentrations (65%, p < 0.05), and the ratio between the areas under the concentration-time curve (78%, p = 0.12). The time needed to reach a minimum inhibitory concentration (MIC) was 30%-40% longer for a collapsed lung than for a ventilated one. In addition, a delay of 10-40 minutes was observed for lung ISF compared with plasma. The mean residence time values (ISF collapsed lung > ISF ventilated lung > plasma) could explain the absence of practically important differences in the time interval with the concentration of cefuroxime exceeding the MICs of sensitive strains (≤4 mg/L). Conclusion: The concentration of cefuroxime in the ISF of a collapsed porcine lung is lower than in a ventilated one; furthermore, its equilibration with plasma is delayed. Administration of the first cefuroxime dose earlier or at a higher rate may be warranted, as well as dose intensification of the perioperative prophylaxis of pneumonia caused by pathogens with higher MICs.

Pharmacokinetics of Macrolide Antibiotics and Transport into the Interstitial Fluid: Comparison among Erythromycin, Clarithromycin, and Azithromycin

Recent research has found higher levels and longer total exposure of azithromycin, a macrolide antibiotic agent, in the interstitial fluid of the skin than in the plasma. This unique distribution is expected to contribute to its antimicrobial activity at the primary infection site. However, it remains unclear whether this characteristic distribution in the extracellular tissue space is common to macrolide antibiotics or if it is azithromycin-specific, with most macrolides largely localized intracellularly. In this study, we investigated pharmacokinetic characteristics of erythromycin and clarithromycin in the interstitial fluid of the skin of rats after intravenous drug administration, and compared the results with our previously reported results on azithromycin. Interstitial fluid samples were directly collected from a pore on the skin using a dissolving microneedle array. We found that the total macrolide concentrations in the interstitial fluid were significantly different among three macrolides. The rank order of the interstitial fluid-plasma concentration ratio was azithromycin (3.8 to 4.9) > clarithromycin (1.2 to 1.5) > erythromycin (0.27 to 0.39), and this ratio was stable after dosing, whereas higher drug levels in the skin tissue than in the plasma were observed for all three macrolides. Our results suggest that lower erythromycin concentrations in the interstitial fluid than in the plasma contributes to the emergence of bacterial resistance in the extracellular tissue space. Monitoring of total macrolide concentrations in interstitial fluid may provide valuable information regarding antimicrobial effects and the emergence of bacterial resistance for the development of an appropriate pharmacokinetics-pharmacodynamics-based dosing strategy.

Fosfomycin: the characteristics, activity, and use in critical care

Fosfomycin (C₃H₇O₄P) is a phosphonic acid derivative representing an epoxide class of antibiotics. The drug is a re-emerging bactericidal antibiotic with a wide range of actions against several Gram-positive and Gram-negative bacteria. Among the existing antibacterial agents, fosfomycin has the lowest molecular weight (138 Da), which is not structurally associated with other classes of antibiotics. In intensive care unit (ICU) patients, severe soft tissue infections (STIs) may lead to serious life-threatening problems, and therefore, appropriate antibiotic therapy and often intensive care management (ICM) coupled with surgical intervention are necessary. Fosfomycin is an antibiotic primarily utilized for the treatment of STIs in ICUs. Recently, fosfomycin has attracted renewed interest for the treatment of serious systemic infections caused by multidrug-resistant Enterobacteriaceae. In some countries, intravenous fosfomycin has been prescribed for various serious systemic infections, such as acute osteomyelitis, nosocomial lower respiratory tract infections, complicated urinary tract infections, bacterial meningitis, and bacteremia. Administration of intravenous fosfomycin can result in a sufficient concentration of the drug at different body regions. Dose modification is not required in hepatic deficiency because fosfomycin is not subjected to enterohepatic circulation.

Use of microdialysis for the assessment of fluoroquinolone pharmacokinetics in the clinical practice

Antibacterial drugs, including fluoroquinolones, can exert their therapeutic action only with adequate penetration at the infection site. Multiple factors, such as rate of protein binding, drug liposolubility and organ blood-flow all influence ability of antibiotics to penetrate target tissues. Microdialysis is an in vivo sampling technique that has been successfully applied to measure the distribution of fluoroquinolones in the interstitial fluid of different tissues both in animal studies and clinical setting. Tissue concentrations need to be interpreted within the context of the pathogenesis and causative agents implicated in infections. Integration of microdialysis -derived tissue pharmacokinetics with pharmacodynamic data offers crucial information for correlating exposure with antibacterial effect. This review explores these concepts and provides an overview of tissue concentrations of fluoroquinolones derived from microdialysis studies and explores the therapeutic implications of fluoroquinolone distribution at various target tissues.

Microdialysis Study of Aztreonam-Avibactam Distribution in Peritoneal Fluid and Muscle of Rats with or without Experimental Peritonitis

The purpose of this study was to investigate aztreonam (ATM) and avibactam (AVI) distribution in intraperitoneal fluid and muscle interstitial fluid by microdialysis in rats, with or without peritonitis, and to compare the unbound concentrations in tissue with the unbound concentrations in blood. Microdialysis probes were inserted into the jugular veins, hind leg muscles, and peritoneal cavities of control rats (n = 5) and rats with intra-abdominal sepsis (n = 9) induced by cecal ligation and punctures. ATM and AVI probe recoveries in each medium were determined for both molecules in each rat by retrodialysis by drug. ATM-AVI combination was administered as an intravenous bolus at a dose of 100-25 mg · kg^-1 Microdialysis samples were collected over 120 min, and ATM-AVI concentrations were determined by liquid chromatography-tandem mass spectrometry. Noncompartmental pharmacokinetic analysis was conducted and nonparametric tests were used for statistical comparisons between groups (infected versus control) and medium. ATM and AVI distribution in intraperitoneal fluid and muscle was rapid and complete both in control rats and in rats with peritonitis, and the concentration profiles in blood, intraperitoneal fluid, and muscle were virtually superimposed, in control and infected animals, both for ATM and AVI. No statistically significant difference was observed between unbound tissue extracellular fluid and systemic areas under the curve for both molecules in control and infected animals. In the present study, intraperitoneal infection induced by cecal ligation and puncture had no apparent effect on ATM and AVI pharmacokinetics in rats.

Transport of Azithromycin into Extravascular Space in Rats

Recent clinical trials showed a prolonged retention of subinhibitory concentrations of unbound azithromycin in the interstitial fluid of soft tissues despite the fact that azithromycin is extensively distributed in tissues. In these clinical trials, interstitial fluid samples were obtained by using the microdialysis method, and it was established that drug concentrations represent protein-unbound drug concentrations. The present study was designed to measure total azithromycin concentrations in the interstitial fluid of the skin of rats by directly collecting interstitial fluid samples from a pore formed on the skin by a dissolving microneedle array. The total azithromycin concentrations in interstitial fluid of the skin were about 4 to 5 times higher than those in plasma throughout the experimental period, and stasis of the azithromycin concentration in interstitial fluid was observed when the concentration of azithromycin in plasma was at the lower limit of quantification. In addition, the skin/plasma concentration ratio transiently increased after dosing (from 4.3 to 83.1). Our results suggest that azithromycin was trapped inside white blood cells and/or phagocytic cells in not only blood but also interstitial fluid, resulting in a high total azithromycin concentration and the retention of its antimicrobial activity at the primary infection site. The stasis of azithromycin in interstitial fluid and skin would lead to long-lasting pharmacological effects (including those against skin infection) at concentrations exceeding the MIC.

Antimicrobial considerations in the perioperative patient

Surgical site infections are among the complications that can be reduced with the timely implementation of appropriate antimicrobial therapy. A 3-D approach to judicious antimicrobial use focuses on the de-escalation of systemic antimicrobial therapy, design of dosing regimens, and decontamination of the surgeon, patient, and environment. De-escalation can be accomplished in part through proper antimicrobial prophylaxis. Dosing regimens should be designed to maximize efficacy and minimize resistance. Decontamination includes disinfection of inanimate surfaces and timely application of appropriate antiseptics at concentrations that maximize efficacy.

Continuous versus short-term infusion of cefuroxime: assessment of concept based on plasma, subcutaneous tissue, and bone pharmacokinetics in an animal model

The relatively short half-lives of most β-lactams suggest that continuous infusion of these time-dependent antimicrobials may be favorable compared to short-term infusion. Nevertheless, only limited solid-tissue pharmacokinetic data are available to support this theory. In this study, we randomly assigned 12 pigs to receive cefuroxime as either a short-term or continuous infusion. Measurements of cefuroxime were obtained every 30 min in plasma, subcutaneous tissue, and bone. For the measurements in solid tissues, microdialysis was applied. A two-compartment population model was fitted separately to the drug concentration data for the different tissues using a nonlinear mixed-effects regression model. Estimates of the pharmacokinetic parameters and time with concentrations above the MIC were derived using Monte Carlo simulations. Except for subcutaneous tissue in the short-term infusion group, the tissue penetration was incomplete for all tissues. For short-term infusion, the tissue penetration ratios were 0.97 (95% confidence interval [CI], 0.67 to 1.39), 0.61 (95% CI, 0.51 to 0.73), and 0.45 (95% CI, 0.36 to 0.56) for subcutaneous tissue, cancellous bone, and cortical bone, respectively. For continuous infusion, they were 0.53 (95% CI, 0.33 to 0.84), 0.38 (95% CI, 0.23 to 0.57), and 0.27 (95% CI, 0.13 to 0.48) for the same tissues, respectively. The absolute areas under the concentration-time curve were also lower in the continuous infusion group. Nevertheless, a significantly longer time with concentrations above the MIC was found for continuous infusion up until MICs of 4, 2, 2, and 0.5 μg/ml for plasma and the same three tissues mentioned above, respectively. For drugs with a short half-life, like cefuroxime, continuous infusion seems to be favorable compared to short-term infusion; however, incomplete tissue penetration and high MIC strains may jeopardize the continuous infusion approach.

Pharmacokinetics of cefuroxime in porcine cortical and cancellous bone determined by microdialysis

Traditionally, the pharmacokinetics of antimicrobials in bone have been investigated using bone biopsy specimens, but this approach suffers from considerable methodological limitations. Consequently, new methods are needed. The objectives of this study were to assess the feasibility of microdialysis (MD) for measuring cefuroxime in bone and to obtain pharmacokinetic profiles for the same drug in porcine cortical and cancellous bone. The measurements were conducted in bone wax sealed and unsealed drill holes in cortical bone and in drill holes in cancellous bone and in subcutaneous tissue. As a reference, the free and total plasma concentrations were also measured. The animals received a bolus of 1,500 mg cefuroxime over 30 min. No significant differences were found between the key pharmacokinetic parameters for sealed and unsealed drill holes in cortical bone. The mean ± standard error of the mean area under the concentration-time curve (AUC) values from 0 to 5 h were 6,013 ± 1,339, 3,222 ± 1086, 2,232 ± 635, and 952 ± 290 min · μg/ml for free plasma, subcutaneous tissue, cancellous bone, and cortical bone, respectively (P < 0.01, analysis of variance). The AUC for cortical bone was also significantly different from that for cancellous bone (P = 0.04). This heterogeneous tissue distribution was also reflected in other key pharmacokinetic parameters. This study validates MD as a suitable method for measuring cefuroxime in bone. Cefuroxime penetration was impaired for all tissues, and bone may not be considered one distinct compartment.

Pharmacokinetic/ pharmacodynamic evaluation of anti-infective agents

Pharmacokinetic/pharmacodynamic modeling has become an extremely important tool in evaluating and optimizing anti-infective therapy. By systematically linking the pharmacokinetic and pharmacodynamic properties of the anti-infective agent, it is possible to make educated decisions about the correct drug to be used, correct dosing regimen and to estimate the probability of success with the selected dose regimen. This article gives an overview of the current pharmacokinetic/pharmacodynamic approaches for anti-infective agents and discusses their use in optimizing drug therapy.

Effect of experimentally induced hypovolemia on ertapenem tissue distribution using microdialysis in rats

Hypovolemia is a common event in critical care patients that may affect drug distribution and elimination. In order to better understand this issue the effect of hypovolemia on the plasma protein binding and tissue distribution of ertapenem was investigated in rats using microdialysis. Microdialysis probes were inserted into the jugular vein and hind leg muscle. Ertapenem recoveries in muscle and blood were determined in each rat by retrodialysis by drug before drug administration. Hypovolemia was induced in 6 rats by removing 40% of the initial blood volume over 30 min. Ertapenem was infused intravenously at a dose of 40 mg kg(-1) over 30 min, and microdialysis samples were collected for 310 min. The unbound concentration profiles in muscle and blood were virtually superimposed in both groups except at early time points. The ratios of the area under the concentration-time curve (AUC) for tissue to the AUC for blood were 0.7±0.2 and 0.8±0.2 for control and hypovolemic rats, respectively. Hypovolemia induced a 40% decrease in the clearance of ertapenem, with no statistically significant alteration of its volume of distribution. This study showed that ertapenem elimination was altered in hypovolemic rats, probably due to decreased renal blood flow, but its distribution characteristics were not. Unbound concentrations of ertapenem in blood and muscle were always virtually identical.

Blood, tissue, and intracellular concentrations of azithromycin during and after end of therapy

Although azithromycin is extensively used in the treatment of respiratory tract infections as well as skin and skin-related infections, pharmacokinetics of azithromycin in extracellular space fluid of soft tissues, i.e., one of its therapeutic target sites, are not yet fully elucidated. In this study, azithromycin concentration-time profiles in extracellular space of muscle and subcutaneous adipose tissue, but also in plasma and white blood cells, were determined at days 1 and 3 of treatment as well as 2 and 7 days after the end of treatment. Of all compartments, azithromycin concentrations were highest in white blood cells, attesting for intracellular accumulation. However, azithromycin concentrations in both soft tissues were markedly lower than in plasma both during and after treatment. Calculation of the area under the concentration-time curve from 0 to 24 h (AUC(0-24))/MIC(90) ratios for selected pathogens suggests that azithromycin concentrations measured in the present study are subinhibitory at all time points in both soft tissues and at the large majority of observed time points in plasma. Hence, it might be speculated that azithromycin's clinical efficacy relies not only on elevated intracellular concentrations but possibly also on its known pleotropic effects, including immunomodulation and influence on bacterial virulence factors. However, prolonged subinhibitory azithromycin concentrations at the target site, as observed in the present study, might favor the emergence of bacterial resistance and should therefore be considered with concern. In conclusion, this study has added important information to the pharmacokinetic profile of the widely used antibiotic drug azithromycin and evidentiates the need for further research on its potential for induction of bacterial resistance.

PK-PD modeling of β-lactam antibiotics: in vitro or in vivo models?

A modified E(max)-pharmacokinetic-pharmacodynamic (PK-PD) model was previously proposed in literature for describing the antimicrobial activity of β-lactam antibiotics based on in vitro experiments. However, bacteria behave differently in vitro and in vivo. Thus, the aims of this study were to model the killing effect of piperacillin (PIP) against Escherichia coli on immunocompromised infected rats using this model and to compare the parameters obtained in vitro and in vivo for the same bacteria/drug combination. The PK-PD parameters determined in vitro and in vivo were as follows: generation rate constant of 1.30 ± 0.10 and 0.76 ± 0.20 h(-1), maximum killing effect of 3.11 ± 0.27 and 1.38 ± 0.20 h(-1) and concentration to produce 50% of the maximum effect of 5.44 ± 0.03 and 1.31 ± 0.27 μg ml(-1), respectively. The comparison between the in vitro and in vivo parameters was not straightforward and had to take into consideration the intrinsic differences of the models involved. So far, the main application of the PK-PD model evaluated is for the comparison of different antimicrobial agent's potency and efficacy, under equivalent conditions.

Challenges in inhaled product development and opportunities for open innovation

Dosimetry, safety and the efficacy of drugs in the lungs are critical factors in the development of inhaled medicines. This article considers the challenges in each of these areas with reference to current industry practices for developing inhaled products, and suggests collaborative scientific approaches to address these challenges. The portfolio of molecules requiring delivery by inhalation has expanded rapidly to include novel drugs for lung disease, combination therapies, biopharmaceuticals and candidates for systemic delivery via the lung. For these drugs to be developed as inhaled medicines, a better understanding of their fate in the lungs and how this might be modified is required. Harmonized approaches based on 'best practice' are advocated for dosimetry and safety studies; this would provide coherent data to help product developers and regulatory agencies differentiate new inhaled drug products. To date, there are limited reports describing full temporal relationships between pharmacokinetic (PK) and pharmacodynamic (PD) measurements. A better understanding of pulmonary PK and PK/PD relationships would help mitigate the risk of not engaging successfully or persistently with the drug target as well as identifying the potential for drug accumulation in the lung or excessive systemic exposure. Recommendations are made for (i) better industry-academia-regulatory co-operation, (ii) sharing of pre-competitive data, and (iii) open innovation through collaborative research in key topics such as lung deposition, drug solubility and dissolution in lung fluid, adaptive responses in safety studies, biomarker development and validation, the role of transporters in pulmonary drug disposition, target localisation within the lung and the determinants of local efficacy following inhaled drug administration.

Antimicrobial chemotherapy and lung microdialysis: a review

Pneumonia is a form of lung infection that may be caused by various micro-organisms. The predominant site of infection in pneumonia is debatable. Advances in the fields of diagnostic and therapeutic medicine have had a less than optimal effect on the outcome of pneumonia and one of the many causes is likely to be inadequate antimicrobial concentrations at the site of infection in lung tissue. Traditional antimicrobial therapy guidelines are based on indirect modelling from blood antimicrobial levels. However, studies both in humans and animals have shown the fallacy of this concept in various tissues. Many different methods have been employed to study lung tissue antimicrobial levels with limited success, and each has limitations that diminish their utility. An emerging technique being used to study the pharmacokinetics of antimicrobial agents in lung tissue is microdialysis. Development of microdialysis catheters, along with improvement in analytical techniques, has improved the accuracy of the data. Unfortunately, very few studies have reported the use of microdialysis in lung tissue, and even fewer antimicrobial classes have been studied. These studies generally suggest that this technique is a safe and effective way of assessing the pharmacokinetics of antimicrobial agents in lung tissue. Further descriptive studies need to be conducted to study the pharmacokinetics and pharmacodynamics of different antimicrobial classes in lung tissue. Data emanating from these studies could inform decisions for appropriate dosing schedules of antimicrobial agents in pneumonia.

Lung microdialysis study of levofloxacin in rats following intravenous infusion at steady state

Lung microdialysis has been used with rats to investigate antibiotic distribution after single-dose administration. However, conducting such experiments after intravenous infusion at steady state would constitute a more convenient alternative, which was evaluated here, using levofloxacin (LVX) as a test compound. Microdialysis probes were inserted in blood and muscle, used as a comparator, between 9:00 a.m. and 11:00 a.m. Intravenous LVX infusion was started 6 h later and maintained until the end of the experiment at a rate of 1.0 mg.h(-1). Lung microdialysis probes were inserted on the morning of the next day. Rats were kept anesthetized during dialysate collection. In vivo probe recoveries were estimated by retrodialysis using a calibrator method, with ciprofloxacin (CIP) as the calibrator. LVX and CIP were analyzed in dialysates by high-performance liquid chromatography. The steady-state tissue-to-blood unbound-drug concentration ratios were 1.00 +/- 0.15 in muscle tissues and 1.06 +/- 0.40 in lungs, suggesting passive distribution of LVX in tissue. Although providing no information on rate of distribution, microdialysis investigations following drug infusion at steady state appear to be an interesting approach for characterization of antibiotic distribution in rat lungs.

Application of basic pharmacokinetic concepts to analysis of microdialysis data: illustration with imipenem muscle distribution

BACKGROUND

Microdialysis studies of antibacterial tissue distribution in critically ill patients have sometimes led to results that were spectacular but inconsistent with basic pharmacokinetic concepts.

OBJECTIVE

To conduct a study of imipenem distribution in the muscle of healthy volunteers and critical care patients in order to compare real-life data with theory.

METHODS

Microdialysis catheters were placed into the quadriceps, and probe recoveries were determined individually in vivo using a retrodialysis-by-drug method. Unbound imipenem concentrations were determined by high-performance liquid chromatography in plasma ultrafiltrates and muscle dialysates, and submitted to noncompartmental pharmacokinetic analysis.

RESULTS

Individual unbound imipenem concentrations in plasma and muscle extracellular fluid were virtually superimposed at any time, both in healthy volunteers and in critical care patients.

CONCLUSION

These new results are not consistent with previously published data obtained in similar conditions by another group, but they are in agreement with results previously obtained in rats, as well as being consistent with basic pharmacokinetic concepts.

Development of amoxicillin enzyme-linked immunosorbent assay and measurements of tissue amoxicillin concentrations in a pigeon microdialysis model

A sensitive ELISA was developed for the detection of amoxicillin (AMX) in serum, urine, and milk. The ELISA used an indirect competitive method produced by coating the plate with ovalbumin conjugated with AMX hapten. Antibodies against AMX-BSA were detected by a goat-antirabbit antibody conjugated with peroxidase. Calibration standard curves ranged from 1.28 ng/mL to 20 microg/mL [IC(50) (inhibition concentration 50%) = 100 ng/mL], and the limits of detection were 1.3, 2.7, and 4.8 ng/mL for urine, milk, and serum, respectively. The intra- and interassay variations were less than 4 and 9.6%. The antibody produced against AMX cross-reacted highly with penicillin G (77%); cross-reacted moderately with ampicillin, oxacillin, and cloxacillin (56.9, 51.4, and 48.8%, respectively); but was considered non-cross-reactive with dicloxacillin (7.4%), cefadroxil (<1%), and cefazolin (<1%). Concentrations of AMX were measured simultaneously in venous blood and muscles by using the developed AMX ELISA in an in vivo microdialysis model designed for pigeons. Following i.m. injection (25 mg/kg), AMX attained a peak blood level of 4.74 +/-0.30 mu g/mL and decreased with a half-life of 2.38 +/-0.16 h. In contrast, measurements in pectoral and femoral muscles exhibited delayed appearances, reduced peak concentrations, and prolonged half-lives of 4.07 +/-0.48 (pectoral) and 3.01 +/-0.26 (femoral) that were significantly different from each other and those in the blood (P < 0.05). Blood protein binding was calculated to be 27.9 +/-5.7%. This study demonstrated the semiquantitative application of a selective AMX ELISA in the first microdialysis procedure for continuous monitoring of drug levels in specific tissues of pigeons and maybe useful for related studies in other poultry species.

Microdialysis study of imipenem distribution in skeletal muscle and lung extracellular fluids of Acinetobacter baumannii-infected rats

The aim of this study was to investigate imipenem distribution in muscle and lung interstitial fluids of rats with Acinetobacter baumannii pulmonary infection. By combining microdialysis in blood and tissues, it was possible to demonstrate that free imipenem concentrations were virtually identical in blood, muscle, and lung.

Pharmacokinetics of unbound linezolid in plasma and tissue interstitium of critically ill patients after multiple dosing using microdialysis

The antimicrobial agent linezolid is approved for the treatment of severe infections caused by, e.g., methicillin-resistant Staphylococcus strains. In order to evaluate the penetration of linezolid into the interstitial space fluid (ISF) of subcutaneous adipose tissue and skeletal muscle of the target population, a microdialysis study was performed with 12 patients with sepsis or septic shock after multiple intravenous infusions. Unbound linezolid concentrations were determined for plasma and microdialysates by use of a validated high-performance liquid chromatography method. Individual compartmental pharmacokinetic (PK) analysis was performed using WinNonlin. In vivo microdialysis was found to be feasible for the determination of unbound linezolid concentrations at steady state in the ISF of critically ill patients. On average, linezolid showed good distribution into ISF but with high interindividual variability. A two-compartment model was fitted to unbound concentrations in plasma with a geometric mean distribution volume of 62.9 liters and a mean clearance of 9.18 liters/h at steady state. However, disposition characteristics changed intraindividually within the time course. In addition, an integrated model for simultaneous prediction of concentrations in all matrices was developed and revealed similar results. Based on the model-predicted unbound concentrations in ISF, a scheme of more-frequent daily dosing of linezolid for some critically ill patients might be taken into consideration to avoid subinhibitory unbound concentrations in the infected tissue. The developed integrated model will be a valuable basis for further PK data analysis to explore refined dosing guidelines that achieve effective antimicrobial therapy in all patients by use of the population PK approach.

Murine models of pneumococcal pneumonia and their applicability to the study of tissue-directed antimicrobials

Animal models have long played a crucial role in the preclinical and postmarketing evaluation of antimicrobial drug efficacy. With the alarming rise of antibiotic resistance in Streptococcus pneumoniae, such models have received increased attention as tools for deriving pharmacodynamic data and for determining the clinical significance of drug resistance. This information helps us to make therapeutic decisions to optimize efficacy and restrict the further promotion of drug resistance. Until now, however, insufficient attention has been paid to the special issues that arise in the evaluation of antimicrobials that are concentrated in tissue relative to serum or plasma. This article reviews the utility of animal models in the evaluation of these tissue-directed antibiotics, citing recent attempts to use murine models of pneumococcal pneumonia to determine the importance of macrolide resistance. A promising new, low-dose aerosol model of pneumonia is briefly presented, as is a discussion of the limitations of existing models.

Lack of effect of experimental hypovolemia on imipenem muscle distribution in rats assessed by microdialysis

The aim of this study was to investigate the influence of hypovolemia on the distribution of imipenem in muscle extracellular fluid determined by microdialysis in awake rats. Microdialysis probes were inserted into the jugular vein and hind leg muscle. Imipenem recoveries in muscle and blood were determined in each rat by retrodialysis by drug before drug administration. Hypovolemia was induced by removing 40% of the initial blood volume over 30 min. Imipenem was infused intravenously at a dose of 70 mg . kg(-1) over 30 min, and microdialysis samples were collected for 120 min from hypovolemic (n = 8) and control (n = 8) rats. The decay of the free concentrations in blood and muscle with time were monoexponential, and the concentration profiles in muscle and blood were virtually superimposed in both groups. Accordingly, the ratios of the area under the concentration-time curve (AUC) for tissue (muscle) to the AUC for blood were always virtually equal to 1. Hypovolemia induced a 23% decrease in the clearance (P < 0.05) of imipenem, with no statistically significant alteration of its volume of distribution. This study showed that imipenem elimination was altered in hypovolemic rats, probably due to decreased renal blood flow, but its distribution characteristics were not. In particular, free imipenem concentrations in blood and muscle were always virtually identical.

Pharmacokinetic modeling of free amoxicillin concentrations in rat muscle extracellular fluids determined by microdialysis

The aim of the present study was to investigate amoxicillin (AMX) distribution in muscle interstitial fluid by microdialysis in healthy, awake rats. Microdialysis probes were inserted into the jugular vein and hind leg muscle. Probe recoveries in each rat were determined by retrodialysis with cefadroxil. AMX was administered as a bolus dose of 50 mg.kg(-1), and microdialysis samples were collected during 180 min. Concentrations of unbound drug in blood and muscle were analyzed simultaneously by a population approach. Simulations were conducted using a hybrid, physiologically based pharmacokinetic model to investigate the potential impact of tissue blood flow on muscle AMX distribution. A two-compartment pharmacokinetic model described adequately the unbound amoxicillin concentration-time profiles in blood and muscle. Muscle AMX distribution equilibrium was rapidly achieved. Consequently, the best results were obtained by considering concentrations in muscle as part of the central compartment. The ratio of the concentration of unbound drug in muscle to that in blood (Rmodel) was estimated to 0.80 by the model, which is close to the mean value obtained by noncompartmental data analysis (Rarea= 0.86 +/- 0.29). Simulations conducted with a hybrid, physiologically based pharmacokinetic model suggest that a muscle blood flow reduction of 30% to 50%, such as could be encountered in critical care patients, has virtually no effect on muscle AMX concentration profiles. In conclusion, this study has clearly demonstrated that AMX distributes rapidly and extensively within muscle interstitial fluid, consistent with theory, and that altered muscle blood flow seems unlikely to have a major effect on these distribution characteristics.

Lung microdialysis--a powerful tool for the determination of exogenous and endogenous compounds in the lower respiratory tract (mini-review)

In vivo measurement of concentrations of drugs and endogenous substances at the site of action has become a primary focus of research. In this context the minimal invasive microdialysis (MD) technique has been increasingly employed for the determination of pharmacokinetics in lung. Although lung MD is frequently employed to investigate various drugs and endogenous substances, the majority of lung MD studies were performed to determine the pharmacokinetic profile of antimicrobials that can be related to the importance of respiratory tract infections. For the lower respiratory tract various methods, such as surgical collection of whole lung tissue and bonchoalveolar lavage (BAL), are currently available for the determination of pharmacokinetics of antimicrobials. Head-to-head comparison of pharmacokinetics of antibiotics in lung revealed high differences between MD and conventional methods. MD might be regarded as a more advantageous approach because of its higher anatomical resolution and the ability to obtain dynamic time-vs-concentration profiles within one subject. However, due to ethical objections lung MD is limited to animals or patients undergoing elective thoracic surgery. From these studies it was speculated that the concentrations in healthy lung tissue may be predicted reasonably by the measurement of concentrations in skeletal muscle tissue. However, until now this was only demonstrated for beta-lactam antibiotics and needs to be confirmed for other classes of antimicrobials. In conclusion, the present review shows that MD is a promising method for the determination of antimicrobials in the lung, but might also be applicable for measuring a wide range of other drugs and for the investigation of metabolism in the lower respiratory tract.

Microdialysis study of imipenem distribution in skeletal muscle and lung extracellular fluids of noninfected rats

The aim of this study was to investigate the imipenem distribution in muscle and lung interstitial fluids by microdialysis in rats and to compare the free concentrations in tissue with the free concentrations in blood. Microdialysis probes were inserted into the jugular vein, hind leg muscle, and lung. Imipenem recoveries in these three media were determined in each rat by retrodialysis by drug period before drug administration. Imipenem was infused intravenously at a dose of 120 mg . kg-1 over 30 min, and microdialysis samples were collected for 150 min. The whole study was conducted with nonhydrated rats (n=4) and hydrated rats (n=6) while the animals were under isoflurane anesthesia. The decay of free concentrations in blood, muscle, and lung with time were monoexponential; and the concentration profiles in these three media were virtually superimposed in both groups. Accordingly, the ratios of the area under the curve (AUC) for tissue (muscle or lung) to the AUC for blood were always virtually equal to 1. Compared to values previously determined with awake rats, clearance was reduced by 2 and 1.5 in nonhydrated and hydrated rats, respectively, but the volume of distribution was unchanged. By combining microdialysis in blood and tissues, it was possible to demonstrate that free imipenem concentrations were virtually identical in blood, muscle, and lung.

Tissue penetration of cefpodoxime into the skeletal muscle and lung in rats

PURPOSE

The aim of this study was to investigate the pharmacokinetics of cefpodoxime in interstitial tissue fluids (skeletal muscle and lung) in rats by microdialysis, and to examine the relationship between free drug levels in plasma and in tissues.

METHODS

Cefpodoxime was administered to anesthetized male Wistar rats as single intravenous bolus of 10 or 20 mg/kg and constant infusion of 260 microg/h with a loading dose. The protein binding of cefpodoxime in rat plasma was determined using ultrafiltration.

RESULTS

The average protein binding of cefpodoxime in rat plasma was 38%. The half-lives in plasma, muscle and lung were similar (approximately 5 h). After constant rate infusion, the free concentrations in the muscle and the lung were almost identical, but lower than total and free plasma concentrations. The data were modeled simultaneously using a two-compartmental body model.

CONCLUSIONS

Free interstitial levels of cefpodoxime in muscle and lung tissue are very similar. Since muscle is more accessible than lung, free muscle concentrations may serve as a good surrogate for unbound concentrations in lung.

Tissue penetration of cefpodoxime and cefixime in healthy subjects

Microdialysis is a technique that allows the measurement of free antibiotic concentrations in different tissues, which are responsible for the antibacterial activity at the infection site. In an open, randomized, 2-way crossover study in healthy volunteers, the muscle penetration of orally administered cefpodoxime (400 mg) and cefixime (400 mg) was compared using microdialysis. The results show that the total plasma concentration-time profiles of each antibiotic were similar; the area under the curve for cefpodoxime was 22.4 +/- 8.7 versus 25.6 +/- 8.5 mg/L*h for cefixime. However, tissue penetration was twice as high for cefpodoxime (area under the curve 15.4 +/- 5.1 mg/L*h) as for cefixime (area under the curve 7.3 mg/L*h). This degree of tissue distribution is consistent with their protein binding of 21% for cefpodoxime and 65% for cefixime. After equilibration, the unbound tissue concentrations of both antibiotics were similar to their unbound plasma concentrations. Pharmacokinetic modeling was applied to describe the pharmacokinetic profiles in plasma and muscle. The study demonstrates that cefpodoxime shows greater tissue penetration than cefixime.

Microdialysis—theoretical background and recent implementation in applied life-sciences

In the past decade microdialysis has become a method of choice in the study of unbound tissue concentrations of both endogenous and exogenous substances. Microdialysis has been shown to offer information about substances directly at the site of action while being well tolerable and safe. The large variety of its field of application has been demonstrated. However, a few challenges have to be met to make this method generally applicable in routine applications. This review will provide an overview over theoretical aspects that have to be considered during the implementation of microdialysis. Moreover, a comparison between microdialysis and other tissue sampling techniques will demonstrate advantages and limitations of the methods mentioned. Subsequently, it will present a critical synopsis of a variety of scientific/biomedical applications of this method with emphasis on the most recent literature, focussing on target tissues while giving examples of substances examined. It is concluded that microdialysis will be of great value in future investigations of pharmacokinetics, pharmacodynamics and in monitoring of disease status and progression.

Interstitial muscle concentrations of rocuronium under steady-state conditions in anaesthetized dogs: actual versus predicted values

INTRODUCTION

The objective of this study was to compare rocuronium effect (C(e)) and peripheral (C(2)) compartment concentrations predicted by pharmacokinetic-pharmacodynamic (PK-PD) modelling with those measured in plasma (C(p)) and in the interstitial fluid of muscle tissue (C(ISF,u)) by microdialysis in anaesthetized dogs.

METHODS

After approval by the Animal Care Committee, eight adult male dogs with a body weight ranging from 7 to 18 kg were anaesthetized with pentobarbital. Each dog received a 2-min rocuronium infusion of 0.15 mg kg(-1) min(-1) followed by a 118-min infusion of 60 microg kg(-1) min(-1) via the right jugular vein. Arteriovenous gradient across the hindlimb was measured at 40, 60, 100 and 120 min. Three microdialysis samples were collected at 40-min intervals. Once the infusion stopped, arterial samples were collected every 2 min for the first 10 min and every 20 min for the next 120 min. Neuromuscular function was monitored using train-of-four stimulation until full recovery. Dogs were then killed and a biopsy of muscle tissue was performed (C(m)).

RESULTS

At steady state, the mean C(ISF,u) value was 1353 ng ml(-1). After correction for the unbound fraction in plasma, the mean C(e,corr) and C(2,corr) were 1681 and 1481 ng ml(-1), respectively. At the terminal sampling point, C(m) was 10-fold higher than C(p).

CONCLUSION

Unbound concentration of rocuronium measured in the muscle interstitial fluid under steady-state conditions confirms that parametric PK-PD modelling gives reliable estimates of effect site concentrations. Rocuronium accumulates in muscle tissue, probably by non-specific protein binding in the interstitial space.

Antimicrobial tissue concentrations

The clinical outcome of anti-infective treatment is determined by both PK and PD properties of the antibiotic. Only the free tissue concentrations of antibiotics at the target site, which are usually lower than the total plasma concentrations, are responsible for therapeutic effect. The free antibiotic concentrations at the site of action are a more appropriate PK input value for PK-PD analysis. The unbound tissue concentrations can be measured directly by microdialysis. Using plasma concentrations overestimates the target site concentrations and its clinical efficacy. The optimal dosing regimens of antibiotics have an impact on patients' outcome and cost of therapy, and reduce the emergence of resistance.

Microdialysis of triamcinolone acetonide in rat muscle

The objective of this study was to compare plasma and muscle concentrations of triamcinolone acetonide (TA) in the rat by microdialysis. Microdialysis experiments were carried out at steady state in rats after an initial I.V. bolus 50 mg/kg of the phosphate ester of TA (TAP) followed by 23 mg/kg/h infusion. In vivo recovery was calculated by retrodialysis. The concentration determined at steady state in microdialysate, corrected for recovery, was 2.73 +/- 0.42 microg/mL compared to 21.9 +/- 2.3 microg/mL in plasma. The pharmacokinetics of TA in plasma was described by an open two-compartment model with a terminal half-life of 2.7 h. The clearance of TA in rats determined by compartmental analysis was 0.94 L/h/kg. The measured microdialysate levels of TA in muscle, corrected for recovery, were comparable to the predicted free drug levels in the peripheral compartment. Protein binding in rat plasma, measured by ultrafiltration, was 90.1%. The microdialysis in vivo recovery in muscle was similar to the in vitro recovery under stirred conditions. The results show the applicability of microdialysis to measure free tissue concentrations of TA in rats.

Determination of norfloxacin free interstitial levels in skeletal muscle by microdialysis

Tissue penetration and distribution of antibiotics are important issues when establishing antibiotic therapies. Free concentrations of antibiotics at the infection site are responsible for bacteria killing effect. The knowledge of the correlation between blood levels and tissue concentrations can be helpful for adequate dosing of these drugs. It was the aim of this study to investigate norfloxacin pharmacokinetics in rats to predict free interstitial levels of the drug, determined by microdialysis, using pharmacokinetic parameters derived from total plasma data. Norfloxacin free tissue and total plasma levels were determined in Wistar rats after administering 5 and 10 mg/kg i.v. bolus doses. Plasma and microdialysis samples were analyzed by high-performance liquid chromatography. Norfloxacin plasma pharmacokinetics was consistent with a two compartments model. A simultaneous fitting of plasma and tissue concentrations was performed using a proportionality factor because norfloxacin free tissue levels determined by microdialysis were lower than those predicted using plasma data. A similar proportionality (f(T)) factor was calculated by the computer program Scientist((R)) for both doses (0.25 +/- 0.08). It can be concluded that it is possible to predict concentration time profiles of norfloxacin in the peripheral compartment based on plasma data using the adequate tissue penetration factor.

Rational dosing of antibiotics: the use of plasma concentrations versus tissue concentrations

At the moment, the most common pharmacokinetic/pharmacodynamic (PK/PD) approaches for anti-infective agents, such as time above MIC, C(max)/MIC and AUC(24)/MIC, rely on plasma concentration as the PK input value and minimum inhibitory concentration (MIC) as the PD input value. However, only the free tissue concentrations of antibiotics at the target site are responsible for the therapeutic effect. Using plasma concentrations frequently overestimates the target site concentrations and therefore clinical efficacy. Microdialysis is a new technique that allows direct measurement of unbound tissue concentrations. Furthermore, a better PD approach, bacterial time-kill curves, can offer more detailed information about the antibacterial activity as a function of time and antibiotic concentration than MICs.

Comparative target site pharmacokinetics of immediate- and modified-release formulations of cefaclor in humans

Optimal dosing of beta-lactam antibiotics aims at maximizing the time at which drug levels in the interstitial space fluid (ISF)--the fluid that surrounds the causative microorganisms at the target site--exceed the minimal inhibitory concentration (MIC). One potentially attractive strategy to achieve this goal is to administer antibiotics as oral sustained-release formulations. The present study was designed to test the hypothesis that sustained-release formulations could lead to a more suitable pharmacokinetic profile in the ISF at the relevant target site. For this purpose, time versus cefaclor concentration profiles attained in the ISF were measured following administration of two formulations, an immediate- (500 mg IR) and a modified-release formulation in two different doses (500 mg MR and 750 mgMR) in a three-way crossover study of healthy male volunteers (n = 12). For the measurement of unbound cefaclor concentrations in the ISF of human skeletal muscle, the in vivo microdialysis technique was employed. For all three formulations, unbound cefaclor concentration in the ISF closely followed individual plasma concentration profiles in a dose-dependent pattern, with ISF to unbound plasma ratios ranging from 0.67 to 0.73. The mean residence time was found to be significantly longer for the MR formulations versus the IR formulation. The data of the present study indicate that time above MIC values at the target site can be substantially prolonged if an antibiotic is administered as a sustained-release product.